GAN-argcPredNet v1.0: a generative adversarial model for radar echo extrapolation based on convolutional recurrent units

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GAN-argcPredNet v1.0: a generative adversarial model for radar echo extrapolation based on convolutional recurrent units
Geosci. Model Dev., 15, 1467–1475, 2022
https://doi.org/10.5194/gmd-15-1467-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

GAN–argcPredNet v1.0: a generative adversarial model for radar
echo extrapolation based on convolutional recurrent units
Kun Zheng1 , Yan Liu1 , Jinbiao Zhang2 , Cong Luo3 , Siyu Tang3 , Huihua Ruan2 , Qiya Tan1 , Yunlei Yi4 , and
Xiutao Ran4
1 Schoolof Geography and Information Engineering, China University of Geosciences, Wuhan 430074, China
2 Guangdong Meteorological Observation Data center, Guangzhou 510080, China
3 Guangdong Meteorological Observatory, Guangdong 510080, China
4 Wuhan Zhaotu Technology Co. Ltd., Wuhan 430074, China

Correspondence: Kun Zheng (zhengk@cug.edu.cn) and Yan Liu (liuyan_@cug.edu.cn)

Received: 24 May 2021 – Discussion started: 6 July 2021
Revised: 25 December 2021 – Accepted: 13 January 2022 – Published: 18 February 2022

Abstract. Precipitation nowcasting plays a vital role in pre-      of the result graph and the evaluation index. The recursive
venting meteorological disasters, and Doppler radar data act       prediction method will produce the phenomenon that the pre-
as an important input for nowcasting models. When using the        diction result will gradually deviate from the true value over
traditional extrapolation method it is difficult to model highly   time. In addition, the accuracy of high-intensity echo extrap-
nonlinear echo movements. The key challenge of the now-            olation is relatively low. This is a question worthy of further
casting mission lies in achieving high-precision radar echo        investigation. In the future, we will continue to conduct re-
extrapolation. In recent years, machine learning has made          search from these two directions.
great progress in the extrapolation of weather radar echoes.
However, most of models neglect the multi-modal character-
istics of radar echo data, resulting in blurred and unrealis-
tic prediction images. This paper aims to solve this problem       1   Introduction
by utilizing the features of a generative adversarial network
(GAN), which can enhance multi-modal distribution mod-             Precipitation nowcasting refers to the prediction and analysis
eling, and design the radar echo extrapolation model GAN–          of rainfall in the target area over a short period of time (0–6 h)
argcPredNet v1.0. The model is composed of an argcPredNet          (Bihlo, 2019; Luo et al., 2020). The important data needed
generator and a convolutional neural network discriminator.        for this work come from Doppler weather radar with high
In the generator, a gate controlling the memory and output is      temporal and spatial resolution (Wang et al., 2007). Relevant
designed in the rgcLSTM component, thereby reducing the            departments can issue early warning information through ac-
loss of spatiotemporal information. In the discriminator, the      curate nowcasting to avoid loss of life and destruction of
model uses a dual-channel input method, which enables it           infrastructure (Luo et al., 2021). However, this task is ex-
to strictly score according to the true echo distribution, and     tremely challenging due to its very low tolerance for time
it thus has a more powerful discrimination ability. Through        and position errors (Sun et al., 2014).
experiments on a radar dataset from Shenzhen, China, the              The existing nowcasting systems mainly include two
results show that the radar echo hit rate (probability of de-      types, numerical weather prediction (NWP) and radar echo
tection; POD) and critical success index (CSI) have an av-         extrapolation (Chen et al., 2020). The widely used optical
erage increase of 21.4 % and 19 %, respectively, compared          flow method several problems, such as its poor capture qual-
with rgcPredNet under different intensity rainfall thresholds,     ity in fast echo change regions, the high complexity of the al-
and the false alarm rate (FAR) has decreased by an average         gorithm and its low efficiency (Shangzan et al., 2017). Since
of 17.9 %. We also found a problem during the comparison           echo extrapolation can be considered a time series image-
                                                                   prediction problem, these shortcomings of the optical flow

Published by Copernicus Publications on behalf of the European Geosciences Union.
GAN-argcPredNet v1.0: a generative adversarial model for radar echo extrapolation based on convolutional recurrent units
1468                                                                                 K. Zheng et al.: GAN–argcPredNet v1.0

method are expected to be solved by using a recurrent neural       movement and evolution of radar echo based on the U-NET
network (RNN) (Giles et al., 1994).                                convolutional network for extrapolation prediction (Ayzel et
   With the continuous development of deep learning, more          al., 2020). PredNet is based on a deep coding framework and
and more neural networks have been applied to the field            adds error units to each network layer that can transmit error
of nowcasting. Forecast models such as ConvLSTM and                signals like the human brain structure (Lotter et al., 2016).
EBGAN-Forecaster show that this new method’s extrapo-              In order to increase the depth of the network and the connec-
lation effect is better than that of optical flow method (Shi      tions between modules, Skip-PredNet further introduces skip
et al., 2015; Chen et al., 2019). However, these models still      connections and uses ConvGRU as the core prediction unit.
have the problem of blurred and unrealistic prediction im-         Experiments show that its effect is better than the TrajGRU
ages (Tian et al., 2020; Xie et al., 2020; Jing et al., 2019).     benchmark (Sato et al., 2018). Although these networks can
One of the main reasons is that radar echo maps are typically      achieve echo prediction, they have problems with both blur-
multi-modal data acquired by multiple sensors and different        ring and producing unrealistic extrapolated images.
stations; some algorithms ignore this feature of radar echo
maps, using the mean square error and mean absolute error          2.2    GAN-based radar echo extrapolation
as the loss function, which is better suited to a unimodal dis-
tribution.                                                         The generative adversarial network (GAN) consists of two
   The paper proposes a generative adversarial network-            parts: a generator and a discriminator (Goodfellow et al.,
argcPredNet (GAN–argcPredNet) network model, which                 2014). GAN can be an effective model for generating images.
aims to solve this problem through GAN’s ability to                Using an additional GAN loss, a model can better achieve
strengthen the characteristics of multi-modal data model-          multi-modal data modeling and each of its outputs will be
ing. The generator adopts the same deep coding–decoding            clearer and more realistic (Lotter et al., 2016). Multiple com-
method as PredNet to establish a prediction model, and uses        plementary learning strategies show that generative adversar-
a new structure of convolutional long short-term memory            ial training can maintain the sharpness of future frames and
(LSTM) as a predictive neuron, which can effectively re-           solve the problem of lack of clarity in prediction (Michael et
duce the loss of spatiotemporal information compared with          al., 2015). In this regard, the extrapolators built a generative
rgcLSTM. The deep convolutional network is used as the dis-        adversarial network to solve the problem of extrapolated im-
criminator to classify the distribution, and the dual-channel      age blur by trying to use this adversarial training to extrapo-
input mechanism is used to strictly judge the distribution of      late more detailed radar echo maps (Singh et al., 2017). Sim-
real radar echo images. Finally, based on the weather radar        ilarly, an adversarial network with ConvGRU as the core was
echo dataset, the generator and the discriminator are alter-       proposed, mainly to solve the problem of ConvGRU’s inabil-
nately trained to make the extrapolated radar echo map more        ity to achieve multi-modal data modeling (Tian et al., 2020).
real and precise.                                                  There are also researchers working on the idea of a four-
                                                                   level pyramid convolution structure that propose four pairs
                                                                   of models to generate an adversarial network for radar echo
                                                                   prediction (Chen et al., 2019). It should be noted that the tra-
2     Related work
                                                                   ditional GAN network has the problem that it uses unstable
2.1    Sequence prediction networks                                training, which will cause the model unable to learn. There-
                                                                   fore, the design of the nowcasting model should be based on
The essence of radar echo image extrapolation is the prob-         a stable and optimized GAN network.
lem of sequence image prediction, which can be solved by
implementing an end-to-end sequence learning method (Shi
                                                                   3     Model
et al., 2015; Sutskever et al., 2014). ConvLSTM introduces
a convolution operation into the conversion of the internal        In this section, we describe the model both overall and in de-
data state of the LSTM, effectively utilizing the spatial in-      tail. Section 3.1 introduces the overall structure and training
formation of the radar echo data (Shi et al., 2015). However,      process of the model. In Sect. 3.2, we describe the structure
the trajectory gated recurrent unit (TraijGRU) has also been       of the argcPredNet generator and focus on the argcLSTM
proposed as a solution (Shi et al. 2017) since the location-       neuron. In Sect. 3.3, we introduce the design of the discrimi-
invariant nature of the convolutional recursive structure is in-   nator and the loss function of the model.
consistent with the natural change motion. A GRU (gated re-
current unit), as a kind of recurrent neural network, performs     3.1    GAN–argcPredNet model overview
a similar function to LSTM but is computationally cheaper
(Group, 2017). Similarly, ConvGRU introduces convolution           Radar echo extrapolation refers to the prediction of the dis-
operations inside the GRU to enhance the sparse connectivity       sipation and distribution of future echoes based on the exist-
of the model unit and is used to learn video spatiotemporal        ing radar echo sequence diagram. If the problem is formu-
features (Ballas et al., 2015). The RainNet network learns the     lated, then each echo map can be regarded as a tensor, i.e.,

Geosci. Model Dev., 15, 1467–1475, 2022                                            https://doi.org/10.5194/gmd-15-1467-2022
GAN-argcPredNet v1.0: a generative adversarial model for radar echo extrapolation based on convolutional recurrent units
K. Zheng et al.: GAN–argcPredNet v1.0                                                                                                                       1469

x ∈ R W×H×C , where W, H and C represent the width, height                                         In Eqs. (2)–(6), ∗ represents convolution operation; ◦ rep-
and number of channels, respectively, and R represents the                                      resents the Hadamard product; σ represents sigmoid nonlin-
feature area. If M represents a given sequence of echo maps                                     ear activation function’ f (t) and g (t) represent the forget gate
and N is used to predict the most likely changes in the fu-                                     and update gate, respectively; and x (t) , h(t) , and C (t) repre-
ture, this problem can be expressed as Eq. (1). This paper                                      sent the input, hidden state, and unit state at time t, respec-
sets the input sequence M and output sequence N to 5 and 7,                                     tively.
respectively.
                                                                                                3.2.2    argcPredNet
     t+1                 t+N       argmax
x̂         , . . ., x̂         = t+1
                                 x , . . ., x t+N                                               The argcPredNet generator has the same structure as Pred-
                                     t+1                t+N        t−M+1            t           Net, which is composed of a series of repeatedly stacked
                               p(x         , . . ., x         |x           , . . ., x )   (1)
                                                                                                modules, with a total of three layers. The difference is
Unlike other forecasting models, GAN–argcPredNet uses                                           that argcPredNet uses argcLSTM as the prediction unit. As
WGAN-gp (Wasserstein generative adversarial network with                                        shown in Fig. 3, each layer of the module contains the follow-
gradient penalty) as a predictive framework. The model                                          ing four units: the input convolutional layer (Al ), the recur-
solves the problem of training instability through gradient                                     rent representation layer (Rl ), the prediction convolutional
penalty measures (Gulrajani et al., 2017). Our model mainly                                     layer (Âl ) and the error representation layer (El ).
includes two parts: generator and discriminator. As shown                                           The recursive prediction layer uses the argcLSTM loop
in Fig. 1, the generator is composed of argcPredNet, which                                      unit, which is used to generate the prediction of the next
is responsible for learning the potential features of the data                                  frame and the input of Âl and Al+1 . The network uses error
and simulating the data distribution to generate prediction                                     calculation, where El will output an error representation, and
samples. Following this, the predicted samples and the real                                     the error representation is passed forward through the convo-
samples are input into the discriminator to make a judgment,                                    lutional layer to become the input of the next layer Al+1 . The
where the real data are judged to be true, and thus the pre-                                    hidden state of the recurrent unit Rlt is updated according to
dicted data are judged to be false. Finally, we use the Adam                                    the output of Elt−1 , Rlt−1 and the up-sampled Rl+1  t . For A ,
                                                                                                                                                               l
optimizer for training the adversarial loss and then update                                     the input of the lowest target, namely A0 , is set to the actual
the parameters of the discriminator, optimize the loss func-                                    sequence itself. When l > 0, the input of Al is the lower er-
tion of the generator once every five updates and complete                                      ror signal El−1 , which results from convolution calculation,
the update of the generator parameters. The algorithm flow                                      linear rectification function (RELU) activation and the max-
is shown in Table 1 (Gulrajani et al., 2017).                                                   imum pooling layer. The complete update rules are shown in
                                                                                                Eqs. (7) to (10). The specific parameter settings of the gener-
3.2         argcPredNet generator                                                               ator are shown in Table 2. The numbers 1, 128, 128 and 256
                                                                                                represent the number of filters used from the first layer to the
3.2.1           argcLSTM                                                                        fourth layer.
                                                                                                      
The internal structure of the argcLSTM neuron used in the                                         t       xt
                                                                                                Al =                                        t
                                                                                                                                               
model is shown in Fig. 2. In order to provide better feature-                                             MAXPOOL RELU CONV El−1
extraction capabilities, the structure contains two trainable                                          if l = 0
gating units: the forget gate f (t) and the input gate g (t) . The                                                                                            (7)
                                                                                                      0
GAN-argcPredNet v1.0: a generative adversarial model for radar echo extrapolation based on convolutional recurrent units
1470                                                                                           K. Zheng et al.: GAN–argcPredNet v1.0

Figure 1. The schematic of the GAN–argcPredNet architecture.

Table 1. GAN–argcPredNet training algorithm flow.

          Algorithm Model uses default values of λ = 10, ncritic = 5, α = 0.0001, β1 = 0.5, β2 = 0.9.
          Parameters: The gradient penalty coefficient λ, the number of critic iterations per generator iteration ncritic , the batch
          size m, Adam hyperparameters α, β1 , β2 , initial critic parameters ω0 , initial generator parameters θ0 .
          1. for i = 1, · · ·epoch do
          2. for t = 1· · ·ncritic do
          3.       for j = 1· · ·m do
          4.         Sample real data x ∼ Pr , latent variable z ∼ pz , a random number  ∼ ∪ [0, 1]
          5.         x̃ ← Gθ (z)
          6.         x̂ ← x + (1 − ) x̃
                                                                        2
                     L(j ) = Dω (x̃) − Dω (x) + λ ∇x̂ Dω x̂ 2 − 1
                                                               
          7.
          8.       end for
                                       m
          9.       ω ← Adam(∇ω m     1 P L(j ) , ωαβ β )
                                                     1 2
                                     j =1
          10.   end for                           n om
          11.   Sample a batch of latent variables zj            ∼ Pz s
                                                          j =1
                                    m
          12.               1
                θ ← Adam(∇θ m
                                    P
                                          −Dω (Gθ (z)), θαβ1 β2 )
                                   j =1
          13. end for

                                                                          Table 2. Generator parameter settings.

                                                                           Components      Name                Filter size        No. of filters
                                                                           Module A        Convolution layer        3×3      (1, 128, 128, 256)
                                                                                           Max pool                 3×3                       /

                                                                           Unit Âtl       Convolution layer        3×3      (1, 128, 128, 256)
                                                                           Module R        Up-sampled               3×3                       /
                                                                                           argcLSTM                 3×3      (1, 128, 128, 256)

                                                                          argcPredNet model, a double-channel convolutional neural
                                                                          network (DC-CNN) network is designed for discrimination.
                                                                          The process is shown in Fig. 4. It is a four-layer convolution
Figure 2. The internal structure of argcLSTM.                             model with a dual-channel input method.
                                                                             The DC-CNN network extracts a pair of images from the
                                                                          three pairs of images, inputs them to the fully connected layer

Geosci. Model Dev., 15, 1467–1475, 2022                                                     https://doi.org/10.5194/gmd-15-1467-2022
GAN-argcPredNet v1.0: a generative adversarial model for radar echo extrapolation based on convolutional recurrent units
K. Zheng et al.: GAN–argcPredNet v1.0                                                                                                 1471

                                                                  Table 3. Discriminator parameter settings.

                                                                      Name            Filter size   Stride   No. of filters   Output size
                                                                      Convolution_1        3×3      2×2                32        48 × 48
                                                                      Convolution_2        3×3      2×2                64        24 × 24
                                                                      Convolution_3        3×3      2×2               128        12 × 12
                                                                      Convolution_4        3×3      2×2               256          6×6

                                                                  The model has the following maximum–minimum loss func-
                                                                  tion (Eq. 13):
                                                                      min max                        
                                                                              V (D, G) = Ex̃∼Pg D (x̃) − Ex∼Pr [D (x)]
                                                                      G   D
                                                                                                               2
                                                                                       + λEx̂∼Px̂ ∇x̂ D x̂ 2 − 1 , (13)
                                                                  where x̃ represents the distribution of generated samples, Pg
Figure 3. Module expansion diagram of layer l at time t.          represents the set of generated sample distributions, x repre-
                                                                  sents the distribution of real samples and Pr represents the
                                                                  set of real sample distributions. The third term is the penalty
                                                                  item of the gradient penalty mechanism. In the penalty term,
                                                                  x̂ represents the actual data and generation of a new sample
                                                                  formed by random sampling between data. Px̂ represents a
                                                                  set of randomly sampled samples. λ is a hyperparameter that
                                                                  represents the coefficient of the penalty term, and the value
                                                                  in the model is set to 10.

Figure 4. The DC-CNN structure.
                                                                  4     Experiments

                                                                  In order to verify the effectiveness of the model, the pa-
through a four-layer convolution transformation and finally       per uses the radar echo data from January to July 2020
generates a probability output through the Sigmoid function,      in Shenzhen, China, to conduct experiments on the four
indicating the possibility that the input image is from a real    models of ConvGRU, rgcPredNet, argcPredNet, and GAN–
image. When the input is a real image, the discriminator will     argcPredNet. All experiments are implemented in Python
maximize the probability, and thus the value will approach        and based on the Keras deep-learning library with Tensor-
1. If the input is a generator-synthesized image, the discrim-    Flow as the back end for model training and testing.
inator will minimize the probability, and thus the value will
approach −1. The specific parameter settings of the discrim-      4.1    Dataset description
inator are shown in Table 3.
                                                                  This experiment uses the radar echo data of Shenzhen, China.
                                                                  The dataset is made up of rain images after quality control.
3.3.2   Loss function
                                                                  The reflectivity range is 0–80 dBZ, the amplitude limit is be-
                                                                  tween 0 and 255, and the data are collected every 6 min, with
The generative adversarial network relies on the distribution     a total of one layer. The height above sea level is 3 km. A to-
of simulated data to generate images. It can retain more echo     tal of 600 000 echo images were collected, of which 550 000
details, thereby realizing the modeling of multi-modal data.      were used as the training set and 50 000 were used as the test
A gradient penalty term is added to GAN–argcPredNet, and          set. Each set of data contained 12 consecutive images. The
the loss function of the discriminator is shown in Eq. (11).      horizontal resolution of the radar echo maps is 0.01◦ (about
                                                                  1 km), the number of grids is 501×501 (i.e., an area of about
                                               2                500 km × 500 km) and the image resolution is 96×96 pixels.
LD = D (x̃) − D (x) + λ ∇x̂ D x̂          2
                                              −1           (11)
                                                                  4.2    Evaluation metrics
The generator has the following loss function (Eq. 12):
                                                                  In order to evaluate the accuracy of the model for precipita-
                                                                tion nowcasting, the experiment uses three evaluation indica-
LG = Ex̃∼Pg D (x̃) − Ex∼Pr [D (x)] .                       (12)   tors to evaluate the prediction precision of the model, critical

https://doi.org/10.5194/gmd-15-1467-2022                                              Geosci. Model Dev., 15, 1467–1475, 2022
1472                                                                                     K. Zheng et al.: GAN–argcPredNet v1.0

Figure 5. CSI index score.                                         Figure 7. FAR index score.

                                                                   Table 4. Rain level

                                                                           Rain rate (mm h−1 )    Rainfall level
                                                                           0 ≤ R < 0.5            None or hardly noticeable
                                                                           0.5 ≤ R < 2            Light
                                                                           2≤ R
K. Zheng et al.: GAN–argcPredNet v1.0                                                                                             1473

Figure 8. Four prediction examples for the precipitation nowcasting problem. From top to bottom: ground truth frames, prediction by GAN–
argcPredNet, prediction by argcPredNet, prediction by rgcPredNet, prediction by ConvGRU.

rainfall level is 30 ≤ R, its leading advantage is the weak-          Table 5. MSE and MSSIM index scores of each model.
est. ArgcPredNet only leads rgcPredNet by a slight mar-
gin, and its performance with rgcPredNet is not as good as                     Name                  MSE × 102 ↓     MSSIM ↑
ConvGRU in the range of 2–5 mm h−1 . For POD indicators,                       ConvGRU                      0.950        0.705
GAN–argcPredNet performs best, and its leading advantage                       rgcPredNet                   0.496        0.724
is more prominent. The performance of argcPredNet is not                       argcPredNet                  0.476        0.790
so outstanding as it is almost the same as argcPredNet, but                    GAN–argcPredNet              0.451        0.833
the index of the two is always better than ConvGRU. For
the FAR score, the performance of GAN–argcPredNet is still
the best, while argcPredNet and rgcPredNet are worse than                In order to compare the prediction results more specif-
ConvGRU in the range of 2–5 mm h−1 , and the score for the            ically, the experiment uses mean square error (MSE) and
rain rate 0.5–10 mm h−1 interval is slightly lower than that of       mean structural similarity (MSSIM) to evaluate the quality
GAN–argcPredNet.                                                      of the generated images (Wang et al., 2004). The MSE and
   To compare the three methods more intuitively, Fig. 8              MSSIM index scores of the images generated by each model
shows the image prediction results of the three models on             are shown in Table 5. ConvGRU has the lowest two indexes.
the same piece of test data.                                          Although the MSE index of rgcPredNet is slightly lower than
   Compared with the other three models, GAN–argcPredNet              that of the argcPredNet and GAN–argcPredNet models, the
generates better image clarity and shows more detailed fea-           MSSIM index of the argcPredNet and GAN–argcPredNet
tures on a small scale. The contrast between the areas                models is 0.066 and 0.109 higher than that of the rgcPred-
marked by the red circle in Fig. 8 is more obvious. GAN–              Net network model, respectively.
argcPredNet made the best prediction of the shape and in-
tensity of the echo. The area selected by the rectangle mainly
shows the echo changes in the northern region within 30 min.          5   Conclusions
Both models correctly predict the movement of the echo to
a certain extent, and the prediction process shown by GAN–            This study demonstrated a radar echo extrapolation model.
argcPredNet is the most complete. In some mixed intensity             The main innovations are summarized as follows. First, the
and edge areas, our model clearly predicts the echo intensity         argcPredNet generator is established based on the time and
information, which clearly shows the effect of the confronta-         space characteristics of radar data. The argcPredNet genera-
tion training.                                                        tor can predict future echo changes based on historical echo
                                                                      observations. Second, our model uses adversarial training
                                                                      methods to try to solve the problem of blurry predictions.

https://doi.org/10.5194/gmd-15-1467-2022                                                 Geosci. Model Dev., 15, 1467–1475, 2022
1474                                                                                        K. Zheng et al.: GAN–argcPredNet v1.0

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                                                                        Luo, C., Li, X., Ye, Y., Wen, Y., and Zhang, X.: A
Disclaimer. Publisher’s note: Copernicus Publications remains              Novel LSTM Model with Interaction Dual Attention
neutral with regard to jurisdictional claims in published maps and         for Radar Echo Extrapolation, Remote Sensing, 13, 164,
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                                                                        Luo, C., Li, X., and Ye, Y.: PFST-LSTM: A Spatio Tempo-
                                                                           ral LSTM Model With Pseudoflow Prediction for Precipi-
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Technology Planning Project of Guangdong Province, China (grant            https://doi.org/10.1109/JSTARS.2020.3040648, 2021.
no. 2018B020207012).                                                    Michael, M., Camille, C., and Yann, L.: Deep multi-scale video
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ence and Technology Planning Project of Guangdong Province                 itation prediction with skip-connected prednet, in: Interna-
(grant no. 2018B020207012).                                                tional Conference on Artificial Neural Networks, 373–382,
                                                                           https://doi.org/10.1007/978-3-030-01424-7_37, 2018.
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https://doi.org/10.5194/gmd-15-1467-2022                                                    Geosci. Model Dev., 15, 1467–1475, 2022
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